Drawing instrument



Jn- 8, 1952 G. T. DOUGLAS DRAWING INSTRUMENT Filed Feb. '7, 1946 INVENTOR G50/PGE' 7.' Ol/GL 195 Kw TP IN @Il EEEMM EEM@.mxlmmwcicfmc:EF.. TP IDEEM P {WWII}IIIIIHHHHMH}:-I: M.

ATTORNEY5 Patented Jan. 8, 1952 l "This invention relates to'drawing' instruments and primarily to Compasses designed for drawing on blackboards or-otherverticalsurfaces. It is frequently necessary, in instructing a group of people in mathematicarphysics and other subjects, to make blackboard illustrationsA involving,y circles, and although various instruments for facilitating the drawing of circles on vertical surfaces` have been proposed, there is notte my knowledge an instrument of this kind on the market, and "it is the universal custom to use makeshift expedients such as string and a piece ofchalk While blackboard drawingsare?I usually merely illustrative and are not required to be exact in their measurements, it is desirable that they be neatly and quickly drawn with approximate accuracy, and the object of this invention is to provide an instrument of simple construction which may be cheaply made, with which circles and circular arcs of varying dimensions may be quickly and accurately drawn on the blackboard with ordinary chalk.

Another object of this invention is to provide an apparatus of this kind wherein the center of the circle may be marked without removing the instrument from the board, and radial lines may also be accurately drawn at measured angles l to each other.

v2--2 of Fig. 1; and

Figs. 3, 4 and 5 are cross sections on the correspondingly numbered lines of Fig. 1.

Referring to the drawings, I indicates a base member which may be made of plastic, Wood or metal, and is preferably of semicircular configuration, as shown. Attached to the bottom of the base piece I are three suction cups 2 of rubber or other similar material arranged so that when the base piece I is pressed against the blackboard the suction cups will adhere to the surface of the blackboard and firmly hold the base I in iixed position.

I `'The upper surface n n with a protractor scale 3 as shown, and co-oper.-

or 'the rpas@ isy provided ating with the scale yis the pointed end 4o'f iiat v-bar or ruler 5. which is pivoted to the base .portion at the middle point of its straight side inY such manner that the axis nof rotation of the ruler 5 corresponds with the center of the pro,- tractor scale 3. A n i v The ruler-5 may be made of-wood, metal or plastic, as desired, andis slotted along the middle as indicated at E tovprovide a path -of move- `ment Vfor thachalk `which is vvcarried.'byf-,the

chalk-carrying guide x1, which; willtele'cr :described. The slot 6 enlarged at the end adjacent the point 4 and its enlarged portion fits against a semicircular collar 8 fastened to the edge of the protractor at the center of its fiat side, as indicated in Fig. 2. This semicircular collar 8 has upper and lower flanges separated by a distance equal to the combined thickness of the base part I and the ruler 5, and is held in place by screws S which pass through holes in the end flange of the collar 8 and project into a circular groove I0, indicated in dotted lines in Fig. 1. The combined action of the screws 9 and the upper flange of the collar 8 hold the ruler 5 firmly in place, but permit its complete rotation about the center of the semicircular collar 8 which, as stated, is coincident with the center of the protractor scale 3.

The ruler 5 is provided with a scale II along one edge and co-operating with the scale I I is an index point I2 of the chalk-carrier 1. The chalk-carrier 1 consists of a metal stamping of the shape shown, the side edges of the stamping being bent around the margin of the ruler which may be undercut along its edges, as indicated at I3 in Figs. 4 and 5.

The chalk is held in a hole I4 in a projecting end portion I5 which is bent away from the face of the ruler, as shown in Fig. 2, a sufficient distance to clear the semicircular collar 8 so that the hole I4 may be adjusted into registry with the center of rotation of the ruler 5.

The operation of the apparatus will be obvious from the foregoing description. The base piece I is pressed against the blackboard at any desired position and the center of the circle can then be readily marked. The chalk-carrier 1 is then adjusted to bring the pointer I2 to a point on the scale corresponding with the radius of the circle to be drawn. The chalk is then placed in the hole I4. A complete circle or an arc of any desired magnitude may be drawn. If radii are to be drawn the chalk guide 'I is set to bring the oenter of the hole u in une with the center of `rotation of the ruler, and while holding the ruler against rotation the chalk guide can be moved radially outward to the desired extent. By means of the protractor scale the angular distance between radii may be measured with all the accuracy required for blackboard demonstrations.

While I ,haver illustrated and described a preferred ebo'diment''of` my invention# s'totbe understood that the invention is not llmited to the precise construction illustrated, but that the same may be variously modied Within the scope Y of the appended claims. Y Y

Iclaim: t 1. In an instrument of the'las'sdsciibelfa base member, means for detach'ablyattachingthe base member to a surface, a guidefbar pivoted to said base member for rotationtlroigh ,an are of 360 degrees, the pivotal connection `beltween Asaid AbaseA member and' the guide' harl inlcluding an arcuate inemberjhavingV an; opening "at one side, the guide'bar'khaving afslot' extending fiongitudineuy e thereof ener "outwardiyf'lfromj the asis of said'jarcuatemember, hthe innerendpor- Y tion of 'saidVl guide "bar having: anarcuate'groove "in Ythe underside thereof concentric 'with'j said arcuate member, vmeans extending f upwardly ifrorn said base member into said arcuate groove, and a chalk-holdermovable l'alongl the'4 guide bar "and i having means for maintaining a pice of `chalkin 'the slot inlthe'guide bar t'as the chalk guide is'f moved* along said 'guide bar' 'and' as the being of larger diameter than the width of said -1s,lot, a guide slidingly mounted on said ruler and having an r extension adapted to overlie the center Y oflsidfh'earing when in its innermost position,

t, to overlie said bearing.

andtantopeningfin said extension in a position "GEORGE TAYLOR DOUGLAS.

REFERENCES CITED v.The,following...references are of record inthe 

